Magnetic-Moment Distribution in NiFe and AuFe Alloys

Abstract

We have determined the magnetic form factors and the corresponding magnetic-moment distributions for ordered ${\mathrm{Ni}}_{3}$Fe and for disordered NiFe and AuFe alloys in an attempt to establish the variation of the individual atomic-moment distributions with composition. The results confirm the individual moments previously determined for NiFe alloys by neutron-diffuse-scattering methods and provide an upper limit of $0.03{\ensuremath{\mu}}_{B}$ for the Au moment in a 25-at.% Fe in Au alloy. A small negative moment density between atoms was found for all of the alloys studied. The symmetry of the moment distributions was determined at the Ni and Fe sites in ordered ${\mathrm{Ni}}_{3}$Fe and for Fe in ${\mathrm{Au}}_{0.75}$${\mathrm{Fe}}_{0.25}$. The Fe moment distribution is nearly spherical in both cases (54% ${T}_{2g}$ in ${\mathrm{Ni}}_{3}$Fe and 56% ${T}_{2g}$ in the AuFe alloy) while the Ni moment distribution is strongly distorted. There seems to be little difference between the individual moment distributions in ordered and disordered ${\mathrm{Ni}}_{3}$Fe. In both, the effective ${T}_{2g}$ character decreases relative to pure Ni. The $\mathrm{Ni} d$-function populations in ordered ${\mathrm{Ni}}_{3}$Fe suggest that this decreasing ${T}_{2g}$ character is associated with the larger size of, and the stronger $d\ensuremath{-}d$ overlap with, the Fe atoms. The $\mathrm{Ni} d(\mathrm{xz})$ and $d(\mathrm{yz})$ functions that are directed toward nearest-neighbor Ni atoms have essentially the same population as in pure Ni while the $d(\mathrm{xy})$ function directed toward nearest-neighbor Fe atoms loses population to $d({x}^{2}\ensuremath{-}{y}^{2})$. Thus, there is a repulsion of spin-up electrons away from the regions of strong overlap.